The objective of this project is to explain the presence of intracisternal A particles (IAP) in mouse preimplantation development stages. Possible explanations include: 1) a fortuitous association, 2) a necessary association with IAP playing a specific developmental role, 3) packaging of an excess gene product as IAP, 4) a primarily oncogenic role. Specific aims will be: 1) to demonstrate immunologically whether IAP from neoplastic cells are immunologically equivalent to those present in early mouse development, 2) to determine if IAP with internal diameters of 30nm and 50nm observed in mouse embryos are immunologically the same, 3) to determine the immunologic relatedness of mouse embryo IAP to C particle group-specific antigens, 4) to examine wild mice embryos for IAP, 5) to inhibit IAP production without inhibiting cellular metabolic processes and to monitor further embryonic development, i.e., can IAP production be separated from preimplantation development?, 6) to determine if IAP particle production can be induced by procedures which induce production of C particles, and 7) if IAP are necessary for development, to determine how IAP function within the embryo, i.e. hybridization to determine if an IAP copy is in the embryonic genome, and assaying for the presence of a functional reverse transcriptase. BIBLIOGRAPHIC REFERENCES: Wiley, L.D. and Calarco, P.G. 1975. The effects of anti-embryo sera and their localization on the cell surface during mouse preimplantation development. Dev. Biol. 47:407-418. Calarco, P.G. and Pedersen, R.A. 1976. Ultrastructural observations of lethal yellow (Ay/Ay) mouse embryos. J. Embryol. Exp. Morph. 35: 73-80.